Process and apparatus for the production of sulphur oxides

ABSTRACT

Sulphur oxides are generated from a liquid stream of hydrogen sulphide by feeding the liquid stream to a vacuum stripper where the stream is contacted with a stripping gas including steam under reduce pressure. Hydrogen sulphide is transferred to the stripping gas, whereby a loaded stripping gas is obtained. Water in the loaded stripping gas is condensed producing a H2S rich stream. The H2S is then burned in the stream to produce a stream rich in oxides of sulphur.

RELATED APPLICATIONS

This application is a continuation of PCT application no.PCT/NL2006/000075, designating the United States and filed Feb. 13,2006; which claims the benefit of the filing date of Europeanapplication no. 05075351.6, filed Feb. 11, 2005; each of which is herebyincorporated herein by reference in its entirety for all purposes.

FIELD

The invention is directed to a process and apparatus for the removal ofhydrogen sulphides from streams containing them, in particular fromwastewater streams, and production of sulphur oxides from said removedhydrogen sulphides.

BACKGROUND

In many industrial production processes a stream containing organicmaterial and sulphate is produced, in particular in production processeswherein organic material and sulphuric acid are used. Examples of suchprocesses are processes which involve the use of sulphuric acid for therelease and hydrolysis of lignocellulose, so that the sugar containinggroups that are thus produced can be used in a fermentation process forthe production of ethanol, lactic acid, citric acid, and the like.Commonly the sulphate in these streams is converted to hydrogensulphide, in particular using anaerobic treatment processes, after whicha step is carried out for removing the hydrogen sulphide.

In the art several processes are known for removing hydrogen sulphidefrom process gases. U.S. Pat. No. 5,928,620, for example, discloses aprocess wherein H₂S is converted to elemental sulphur.

SUMMARY

The present invention seeks to provide an efficient process for removingH₂S from process streams. At the same time, the process should make itpossible that the removed H₂S may be converted into useful sulphur oxidecompounds.

It was found that this object can be met by removing H₂S from processstreams, in particular from liquid process streams, more particularlyaqueous process streams, by vacuum stripping followed by burning of theremoved H₂S. Thus in one embodiment, the present invention is directedto a process for the removal of hydrogen sulphide from a liquid stream,comprising the steps of

-   -   feeding said liquid stream to a stripper;    -   contacting said liquid stream in said stripper under reduced        pressure with a stripping gas, which stripping gas comprises        steam that is generated in said stripper, whereby at least part        of said hydrogen sulphide is transferred to said stripping gas,        whereby a loaded stripping gas is obtained;    -   subjecting said loaded stripping gas from said vacuum stripper        to a step wherein water is condensed, thus producing a H₂S rich        stream; and    -   burning H₂S in said H₂S rich stream, preferably using air, thus        producing a stream rich in oxides of sulphur.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and other features and advantages of the present inventionwill be more fully understood from the following description of anillustrative embodiment taken in conjunction with the following drawingin which:

FIG. 1 schematically depicts an embodiment utilizing two strippers.

DETAILED DESCRIPTION

In accordance with the present invention an aqueous solution of sulphidemay be used. Aqueous hydrogen sulphide containing solutions mayoriginate e.g. from anaerobic bioreactors, in which sulphur compounds(e.g. sulphate, sulphite, thiosulphate, certain amino acids, etc.), areconverted to sulphide, while organic compounds are used as an electrondonor. Also H₂, electricity and redox mediators can be used as electrondonor. The use of aqueous sulphide containing streams in accordance withthe present invention is particularly advantageous, because the gas thatis produced in the vacuum stripper, comprises water vapour and hydrogensulphide. It was found that the steam in this gas stream can becondensed relatively easily, whereby a gas stream with a high hydrogensulphide concentration is obtained.

One particular type of bioprocesses are anaerobic biological acidifyingprocesses, in which the organic material is mainly converted into fattyacids, which fatty acids are in turn not converted to methane and CO₂.By result, the fatty acids accumulate and the pH drops. A low pH isfavourable for subsequent sulphide removal, as can be explained by thefollowing reaction equations:S₂ ⁻+H⁺

HS⁻  (1)HS⁻+H⁺

H₂S  (2)

At increased H⁺ concentrations, the equilibrium of (2) shifts to theright hand side and the H₂S concentration increases. By result, the H₂Scan be transferred more easily to the stripping gas. Thus acidifyingbioprocesses are preferred according to the present invention. For thistype of processes it was found that the suitable pH is preferably from 6to 6.9, in particular about 6.5.

The contacting of the stripping gas and the H₂S containing liquid may becarried out in various ways. The stripping gas may be directed throughthe liquid in the form of bubbles. It is also possible to have thegaseous phase as the continuous phase and finely divide the liquid, e.g.by spraying the liquid from the top of the stripping column. In thelatter case, it is usually preferred to have column packings present inthe stripper, in order to increase the contact area between strippinggas and liquid. Preferred packings are Pall rings and/or saddle rings.Usually, when a strip gas is to be added to the stripper, the gas is fedat the bottom and the liquid at the top.

DE-A-376 633 describes a vacuum stripping method which involves heatingunder vacuum. Such a method is disadvantageous because it requires moreenergy. Furthermore, this method is not suitable for treating a liquidfeed stream that originates from a bioreactor and comprises livingbacteria, these bacteria will generally not survive such hightemperatures.

In accordance with the present invention, the stripping is carried outin vacuum, viz. under reduced pressure, i.e. at pressures lower thanatmospheric, typically lower than 0.5 bara. Preferably the pressure inthe stripper is from 0.01 to 0.2 bara, more preferably from 0.05 to 0.1bara. These low pressures can easily be applied by using vacuum pumps,which are placed down stream of the stripper, in combinations withsuitable restriction upstream of the low pressure segment. As a resultof these low pressures, the water which contains the sulphides maycommence boiling at already very low temperatures, e.g. at about 30° C.It was found that by using very low pressures, H₂S can be removedwithout heating and bacteria can survive the vacuum stripping step.

By employing a lower pressure, water evaporates from the liquid and thethus formed steam enters the gas phase, where it will (co-)act as astrip gas. Another advantage of the removal of water from the liquidphase is that the concentration of H₂S in the liquid phase increases, asresult of which the transfer of H₂S to gaseous phase is furtherimproved. Preferred temperatures for operating the stripper are from 20to 80° C., more preferably from 25 to 35° C., typically around 30° C.

The mean residence time of the stripping gas in the stripper istypically from 1 to 100 seconds.

One of the advantages of the present invention is that theconcentrations of the loaded strip gas, viz. the gas which contains theH₂S stripped from the liquid phase, may be relatively high. Typically,the loaded stripping gas comprises 5-40 wt. % H₂S, preferably 25-35 wt.% H₂S on a dry gas basis. High concentrations of H₂S are particularlydesirable because the H₂S is to be burnt, as will be explained in moredetail hereinbelow. Although the burning of H₂S is exothermic, theamount of heat produced is too low at low H₂S concentrations, thusrequiring the addition of extra fuel, which is not desirable from aneconomic point of view. Thus operating at a H₂S concentration that is ashigh as possible is desirable. At H₂S concentrations above 4.5 wt. % inair, H₂S can burn in a self supporting flame.

In addition to the steam that is produced in the stripper, extrastripping gas may added to the bottom of the stripper. This extra streamof stripping gas may be e.g. air, which under certain circumstances hasadditional advantages, as will be explained in more detail hereinbelow.It is also possible to apply this stream of air in a separate stripper,not necessarily a vacuum stripper, which is in line with the vacuumstripper.

In a subsequent step, the loaded stripping gas is subjected to a stepwherein the water content of the gas is reduced, e.g. by means of acondenser. In this way a dry H₂S rich stream is produced. Apart from H₂Sand depending on the upstream process, the dry H₂S rich gas may compriseother gases, such as CO₂. Typically the dry gas further comprises 95-60wt % CO₂, preferably 65-75 wt. % CO₂, based on dry gas.

The sulphide containing liquid, from which the sulphide is recovered inaccordance with the present invention, may originate from varioussources, such as from a wastewater treatment process (such as tannerywastewater); or from a process for the production of fermentationproducts (such as ethanol, lactic acid, citric acid, etc.).

The (dry) H₂S rich stream produced in accordance with the presentinvention may be processed further. For instance, it is possible toproduce elemental sulphur (S_(x)) from H₂S by means of the well knownClaus process.

In a preferred embodiment, however, the H₂S rich gas stream is convertedto a stream rich in oxides of sulphur, such as SO₂ and/or SO₃. Usuallythis is done by reacting the H₂S with oxygen, whereby the H₂S is thuseffectively burnt, whereby normally SO₂ is formed. SO₂ may consequentlybe further oxidized to SO₃ by means of a suitable catalyst. In a nextstep the SO₃ may be dissolved in water, forming H₂SO₄. The burning ofthe H₂S is preferably carried out using air. As indicated hereinabove,it is even more preferred if the air is entered at the vacuum stripperand/or in the second or further separate stripper, so that it maycontribute in stripping action.

In a preferred embodiment of the invention, the liquid feed streamoriginates from an anaerobic biological wastewater treatment reactor.Such a system effectively converts low value energy of organic compoundsthat are dissolved in wastewater to high value energy, such as the heatfrom burnt H₂S. This high value energy can be used for differentpurposes, for example to reduce costs. Thus, this embodiment of thepresent invention provides in effect a biological heat pump, in whichlow value energy are converted by using mechanical energy (thecompressor) into high value energy.

FIG. 1 shows schematically an embodiment in which two strippers are usedin accordance with the present invention. In this embodiment wastewaterenters anaerobic reactor 1, in which acidification and sulphatereduction takes place. The liquid effluent is passed to separator 2,from which sludge is returned to reactor 1. The effluent is passed to ananaerobic post treatment process, which is carried out in reactor 3. theliquid effluent of the reactor 3 is passed to stripper 4, which is fedby air at the bottom. The gaseous stream which leaves stripper 4 is richin air (oxygen) and also contains considerable amounts of H₂S and issubsequently fed to H₂S burner 6, where this gas stream is used as anoxygen source. Another liquid effluent from reactor 1 is fed directly tovacuum stripper 5, which produces a H₂S rich stream that is fed to theburner, where it is burnt using at least part of the oxygen coming fromstripper 4. Optionally extra air may be fed to burner 4, if necessary.It is also possible to place stripper 4 before post treatment reactor 3.

In a specific embodiment of the present invention, the stream rich inoxides of sulphur that is produced by oxidizing the sulphides iscontacted with water, thus producing a sulphuric acid rich stream, whichsulphuric acid rich stream is fed to a step wherein it is contacted withbiomass, thus producing a stream rich in monosaccharides and/orpolysaccharides, which stream rich in monosaccharides and/orpolysaccharides is subsequently subjected to a fermentation step,whereby fermentation products are formed and whereby a sulphate richstream is formed, which sulphate rich stream is at least in partconverted to hydrogen sulphide in said liquid stream, which liquidstream is fed to said stripper.

In wastewater treatment processes, as well as in fermentation productionprocesses, usually one or more alkaline substances are added to theprocess stream to neutralize acids that are formed at some stage inthese processes. The present inventors have found that these alkalinesubstances, when used, are preferably selected from Mg(OH)₂, NaOH andKOH. Ca(OH)₂ is less preferred, because it may lead to undesiredprecipitation in the stripping column. Mg(OH)₂ is particularly preferredwhen (baker's) yeast is used, since it is more sensitive to Na⁺ and K⁺.

Another application of the present invention is the treatment of tailgas from a Claus plant. In a Claus process oxides of sulphur (SO_(x))are produced. This SO_(x) may be absorbed in water and the aqueousstream thus obtained can be treated in the same way as the sulphurcompound containing wastewater streams as described herein above. Thesulphur compounds are converted to sulphide, which is subsequentlystripped in the vacuum stripper in accordance with the presentinvention. The sulphide can be fed to the burner in the Claus process.

When the process of the present invention is used in producingfermentation products (e.g. ethanol) from lignocellulose, it may beadvantageous to take an acidic feed stream that is obtained from thehydrolysis step (by membrane extraction), which is carried out usingsulphuric acid and bypass the fermentation reactor. This sulphuric acidstream is then passed directly to the acidification reactor, or evendirectly to the vacuum stripper because this results in lowering the pHin the stripper, which is favourable for the stripping action, asexplained above. The sulphuric acid is maintained in the liquid effluentand fed back to the bioreactor, where it may be converted into sulphide.Thus, according to this preferred embodiment, a relatively clean, acidicwastewater stream is fed to the vacuum stripper, preferably to the topof the vacuum stripper.

The present invention will now be illustrated by the followingnon-limiting examples.

EXAMPLE 1

To a biological acidification reactor of 5 dm³ was added a syntheticwastewater stream comprising sucrose, yeast extract and sodium sulphate.The amount of sulphur from sulphate was 460 mg S/dm³ and the sucroseamount was 3200 mg/dm³. The pH in the bioreactor was kept constant byfeeding NaOH solution using a pH-stat. The effluent of the reactor wasallowed to flow to a settling tank (5 dm³) where the sludge was allowedto settle and the liquid was removed. The sludge was returned to thereactor. The liquid effluent was fed to a stripping column (1.5 dm³),which was operated at a pressure of 0.08 bara and a temperature of 30°C. The liquid effluent contained only 10 mg sulphide/dm³. The gas wasremoved using a membrane pump. No external stripping gas was used.

The sucrose was converted mainly into acetic acid and the sulphatedisappeared. The sulphide formed from the sulphate could be removed for98 wt. % in the vacuum stripper, despite the low H₂S concentration inthe liquid. The gas from the stripper was subsequently dried bycondensing the water. After drying it contained 30 wt. % H₂S and 70 wt.% CO₂.

It was found that a pH of 6.5 in the bioreactor was optimal. A lower pHresulted in a lower biological activity and a higher pH resulted in alower stripping efficiency.

After mixing it with air, a gas resulted comprising 9 wt. % H₂S and 14wt. % O₂.

EXAMPLE 2 Reference

Example 1 was repeated. Again, the liquid effluent that was fed to thestripper contained 10 mg hydrogen sulphide per dm³ and the pH was 6.5.This time, however, an ordinary (atmospheric) stripper was used,employing N₂ as the stripping gas. The dry gas produced contained only0.25 wt. % H₂S.

1. A process for the production of sulphur oxides from a liquid streamcomprising hydrogen sulphide, comprising the steps of: feeding saidliquid stream to a vacuum stripper; contacting said liquid stream insaid stripper under reduced pressure with a stripping gas, whichstripping gas comprises steam that is generated in said stripper,whereby at least part of said hydrogen sulphide is transferred to saidstripping gas, whereby a loaded stripping gas is obtained; subjectingsaid loaded stripping gas from said vacuum stripper to a step whereinwater is condensed, thus producing a H₂S rich stream; and burning H₂S insaid H₂S rich stream, thus producing a stream rich in oxides of sulphurwherein said process forms part of a wastewater treatment process; of aprocess for the production of fermentation products; or of a Clausprocess.
 2. The process according to claim 1, which optionally comprisesa further stripper that may be operated under atmospheric conditions. 3.The process according to claim 2, wherein said stripping gas furthercomprises air and/or CO₂ that is fed to said vacuum stripper, to saidfurther stripper if present, or both.
 4. The process according to claim2, wherein air is used for burning H₂S, which air is fed as a strippinggas to said vacuum stripper, optionally to said further stripper ifpresent, or both.
 5. The process according to claim 1, wherein saidloaded stripping gas from said vacuum stripper comprises 5-40 wt. % H₂S,based on dry gas.
 6. The process according to claim 1, wherein saidloaded stripping gas from said vacuum stripper further comprises 95-60wt % CO₂, based on dry gas.
 7. The process according to claim 1, whereinwater from an anaerobic biological wastewater treatment reactor istreated.
 8. The process according to claim 1, wherein water from ananaerobic acidification reactor is treated.
 9. The process according toclaim 1, wherein said stream rich in oxides of sulphur is contacted withwater, thus producing a sulphuric acid rich stream, which sulphuric acidrich stream is fed to a step wherein it is contacted with biomass, thusproducing a stream rich in monosaccharides and/or polysaccharides, whichstream rich in monosaccharides and/or polysaccharides is subsequentlysubjected to a fermentation step, whereby fermentation products areformed and whereby a sulphate rich stream is formed, which sulphate richstream is at least in part converted to hydrogen sulphide in said liquidstream, which liquid stream is fed to said stripper.
 10. The processaccording to claim 1, wherein said stripper is filled with pall rings orsaddle rings.
 11. The process according to claim 1, wherein the meanresidence time of said stripping gas is from 1 to 100 seconds.
 12. Theprocess according to claim 1, wherein the pressure in said stripper isfrom 0.01 to 0.2 bara.
 13. The process according to claim 1, wherein analkaline substance is added in a step prior to the vacuum strippingstep.
 14. The process according to claim 13, wherein the alkalinesubstance is Mg(OH)₂.
 15. The process according to claim 1, wherein aclean, acidic wastewater stream is fed to the vacuum stripper.
 16. Theprocess according to claim 15, wherein the clean, acidic wastewaterstream is fed to top of the vacuum stripper.
 17. The process accordingto claim 1, wherein the H₂S rich stream comprises air.
 18. The processaccording to claim 1, wherein said loaded stripping gas from said vacuumstripper comprises 25-35 wt. % H₂S, based on dry gas.
 19. The processaccording to claim 1, wherein said loaded stripping gas from said vacuumstripper further comprises 65-75 wt. % CO₂, based on dry gas.
 20. Theprocess according to claim 1, wherein the pressure in said stripper isfrom 0.05 to 0.1 bara.